The present invention relates to Ziegler-Natta type catalyst compositions for use in the polymerization of ethylene and mixtures of ethylene with one or more C4-C8 α-olefins having improved high temperature polymerization properties. More particularly, the present invention relates to such catalyst compositions that are self-limiting or auto-extinguishing, thereby avoiding polymer agglomeration, operability problems, and/or reactor sheeting, chunking or fouling due to localized overheating.
Catalysts which produce broad molecular weight distributions and high molecular weight tails are desirable for use in both slurry and gas phase polymerization processes, to produce improved products, especially HDPE blow molding resins, where resin swell (caused by high molecular weight chains) is important. However, the production of these polymers with very high molecular weight resin fractions, has been difficult due to reactor operability issues.
Gas phase polypropylene (PP) and polyethylene (PE) polymerization processes are highly exothermic, generating large amounts of heat as the polymerization occurs. One feature of a catalyst system utilized in these polymerization processes is known as “activation energy” and governs the rate at which the polymerization reaction proceeds as the polymerization temperature increases. Catalyst systems in which the rate increases are said to have “positive activation energy”, where the rate decreases, “negative activation energy”.
In a gas phase polymerization process, the polymerization reactor is cooled by the circulating monomer gasses to maintain a steady operating temperature. However, if the temperature of a growing resin particle approaches the sticking/melting point of the resin, resin sheeting on the reactor walls may occur. Growing resin particles are especially susceptible to overheating if they accumulate at the reactor walls, thereby losing heat-transfer with the circulating monomer gasses, and remaining in close contact with respect to each other. In such instances, particle-particle fusion may occur, followed by reactor sheeting, which, in turn, could cause reactor shutdown.
Catalyst systems to minimize or prevent reactor sheeting have been developed for use in propylene polymerization reactions. Such catalyst systems possess mitigating chemical features (e.g., temperature-dependent decomposition of an attached ligand to give a poison) which shut down polymerization when the temperature becomes excessive. That is, such systems result in a negative activation energy, slowing down the polymerization reaction as reaction temperature increases. Alternative known systems use one or more reagents external to the catalyst composition, commonly referred to as Self Limiting Agents (SLA), to slow or deactivate the polymerization reaction. SLAs have been used successfully in propylene polymerization and co-polymerization reactions; for example, SLAs described U.S. Pat. Nos. 7,678,868, 7,381,779 and 7,491,670. The operating temperature for PP polymerization is 65 to 80° C., and the melting point of the resin is about 165° C., giving an 85 to 100° C. temperature span in which an SLA may operate. Generally, polypropylene SLAs shut down the polymerization reaction at an active site when the temperature of the active site reaches about 90° C.
SLAs, however, have not previously been used successfully in two-stage ethylene polymerization or copolymerization reactions or single stage ethylene polymerization reactions operating at high polymerization temperatures. For a two-stage PE process, the polymerization temperatures are about 75 to 95° C. in the first reactor and between 100° C. and 112° C. in the second reactor, while the melting point of the PE resin produced is about 125-135° C. and the sticking temperature, i.e. the temperature at which granular particles begin to adhere to each other, is about 120 to 125° C. For a single stage process, particularly one producing high density polyethylenes, the reaction temperature will also be greater than 100° C. to as high as 112° C. Thus, for such PE polymerization reactions, there is only a 15-25° C. temperature span in which SLAs may function. The use of SLAs are further complicated in two-stage PE polymerization reactions utilizing catalyst that form a very high molecular weight fraction. Specifically, catalysts used in such PE reactions may contain both titanium-based and hafnium and/or zirconium-based active sites or multiple sites based on titanium. The hafnium, zirconium and titanium active sites of the PE catalysts exhibit differing sensitivity to poisoning further complicating the use of SLAs in PE reactions. For example, U.S. Pat. No. 7,393,910 discloses the use of an alkyl or aryl ester of an aliphatic or aromatic carboxylic acid as a self limiting agent for a single-stage polyethylene polymerization. The disclosed esters poison active catalyst sites at temperatures between 90-100° C. and, therefore, would not be useful in producing PE in a two-stage reactor system.
Thus, there remains a need for an SLA for use in two-stage PE reactions, and particularly for use in production of PE with high molecular weight fractions, which deactivates the PE polymerization reaction at temperatures greater than 112° C. and preferably completely deactivating the catalyst at temperatures greater than 125° C. A need further exists for such SLAs which do not impact the properties of the resulting polymer. In particular, there is a need for an SLA which does not negatively impact the formation or properties of the high molecular weight fraction of PE produced in a two-stage reactor.